Embodiments of the present invention relates to semiconductor devices and methods of fabricating the same. More specifically, embodiments of the present invention are directed to a semiconductor device such as a phase change memory device and a method of fabricating the same.
With the advance in electronic industries such as mobile communications or computers, there is a requirement for semiconductor memory devices to have characteristics such as a high read/write speed, non-volatility, a lower operating voltage. However, recently developed memory devices (e.g., SRAM devices, DRAM devices or flash memory devices) do not meet these characteristics.
For example, a unit cell of a DRAM device includes one capacitor and one transistor configured to control the capacitor. Accordingly, the DRAM device has a larger unit cell area than a NAND flash memory device. Further, because the DRAM device includes the capacitor in which data is stored, it is a kind of volatile memory device that requires a refresh operation. While an SRAM device has a high operation speed, it is also a kind of volatile memory device. Because a unit cell of the SRAM device includes six transistors, the SRAM device has a considerably large unit cell area. While a flash memory device is a kind of non-volatile memory device and provides the highest integration density (especially, in case of a NAND flash memory device), it is well known that the flash memory device has a low operation speed.
Accordingly, memory devices having a high read/write speed, which are non-volatile, which do not require refresh operations, and which have a low operation voltage are being studied. A phase change random access memory (PRAM) is becoming attractive as one of the next generation memory devices capable of meeting the above-mentioned requirements. For example, because a PRAM device can rewrite data 1013 times or more, it has advantages as follows: (1) long lifespan; and (2) higher operation speed than 30 nanoseconds.
The data stored in a memory cell of a PRAM may be read out by sensing the resistance change caused by change of a crystalline state of a phase change material layer. The crystalline state of the phase change material layer is dependent on heating temperature and heating time of the phase change material layer. In the PRAM, a state of the phase change material layer is changed to a desired state by controlling current flowing at the phase change material layer and Joule's heat generated by the current. As well known, the Joule's heat (Q) may be given by Joule's Law.Q∝I1Rt  [Equation 1]
Resistance (R) is a fixed parameter that is dependent on a kind of material or fabricating process, whereas time (t) and current (I) are parameters for the operation of products and externally controllable parameters. Thus, resistance of a portion where the phase change material layer is heated is necessarily increased to heat the phase change material layer up to a required temperature while minimizing power dissipation.
From a standpoint of a semiconductor memory device including a phase change random access memory, an increase in operation current (Ion) is required to achieve a high integration and a high speed operation. Resistance elements having an effect on the operation current of the semiconductor memory device are metal resistance, contact resistance, and active region resistance. These resistance elements decrease the operation current of the semiconductor memory device to degrade operation characteristics of the semiconductor memory device.